The invention relates to a mode coupler for monopulse applications in an antenna feed system for obtaining angle deviations in azimuth and elevation. More particularly, the present invention relates to a mode coupler having a main wave guide for electromagnetic waves in which a plurality of wave modes of the electromagnetic waves can be propagated, a plurality of mode-selective input and/or output coupling ports, each formed of standard wave guides in which only the H.sub.10 mode can propagate mounted on the main wave guide, and a separator plate disposed in the main wave guide to convert the (H.sub.11 +E.sub.11) mode in the main wave guide into two phase-opposed waves which is coupled via a coupling loop into a second wave guide mounted laterally with respect to the main wave guide, and with the front edge of the separator plate serving as a reflector for the H.sub.01 mode. Such a mode coupler is known, for instance, from published German Patent Application DE 36 04 432 A1. A similar mode coupler arrangement is described in published German Patent Application DE 36 04 431 A1.
The dimensions of the main wave guide in the aforementioned known mode couplers are selected to be large enough so that all the relevant wave guide modes of the electromagnetic waves can be propagated. However, the dimensions are not so large that undesirable wave guide modes are propagated that could lead to erroneous received signals. By mode-sensitive coupling out of the electromagnetic waves having the H.sub.10 and H.sub.20 modes, signals can be obtained for production of a sum diagram and a differential diagram (direction finding diagram) in the elevation direction, as is known. The end of the main wave guide is connected to additional wave guide structure reduced in stages down to a standard wave guide format. The electromagnetic waves having the H.sub.10 mode are coupled out at the end of the wave guide train in a straight line at a location where the main wave guide has been reduced in stages down to the standard wave guide format. The electromagnetic waves having the H.sub.20 mode are coupled out by a laterally mounted wave guide. The electromagnetic waves having the (H.sub.11 +E.sub.11) mode, furnishing the differential signal diagram representing the angle deviations in the azimuth, are converted in the aforementioned known mode coupler in the main wave guide by a separator plate into two phase-opposed wave guide waves, as is known in principle from European Patent 0 061 576, corresponding to U.S. Pat. No. 4,473,828. The energy of the electromagnetic waves having the (H.sub.11 +E.sub.11) mode then is selectively coupled out from the main wave guide and coupled into another laterally mounted wave guide by a coupling loop.
The similarly designed mode coupler for monopulse applications in an antenna feed system known from German Patent 36 04 431 A1 also is used to obtain angle deviations in azimuth and elevation and comprises a main wave guide in which a plurality of modes can be propagated. A plurality of mode-selective in and/or out coupling ports are mounted on the main wave guide. A further in and/or out coupling port for in/out coupling a mode orthogonal to the sum mode is embodied as a simple wave guide mounted on the main wave guide, in which only the H.sub.10 fundamental wave type can be propagated. A metal reflector is introduced into the main wave guide and reflects the orthogonal mode into the wave guide mounted on it.
Other mode couplers for monopulse applications are described for instance in the textbook entitled Radar Handbook by Skolnik, publ. by McGraw-Hill 1970, Chapter 21, pp. 18 ff, and in European Patents 0 061 576 and 0 041 077.
However, the disadvantage of these last-mentioned known couplers is in that to obtain one of the two deviation signals, they use an out coupling arranging which is complicated to produce and includes an EH-plane junction (magic-T). In the arrangement described in the Skolnik textbook, the differential signal formed by the (H.sub.11 +E.sub.11) mode appears for instance at one output of the EH-plane junction, while a component of a sum signal can be coupled out at a second output. This coupled out component must be joined by a special joining apparatus to the sum signal to be coupled out at the actual summation gate, which necessitates further engineering effort.